Liner-cementing apparatus

ABSTRACT

A dual-plug-cementing device has a first plug for traveling down the operating string ahead of the cement which engages a liner wiper at the bottom of the operating string. Both wiper and plug in sealing engagement travel ahead of the cement to the cementing region. An elongated portion of the plug strikes a stop in the cementing region permitting the wiper to continue downward movement relative to the plug and break the seal permitting the cement to flow past the plug.

[ 1 Jan. 18,1972

3,364,996 1/1968 Brownm.............................166/208X [54] LINER-CEMENTING APPARATUS Primary Examiner-David H. Brown k n a B k a O m P7 m 0 7 7 m mT n 0 t Rm 0 MH 1. m MB n m n e v n 1 U 7 Attorney-Arnold and Durkee, Torn Arnold, Donald C. Roylance, Walter Kruger, Bill Durkee, Frank S. Vaden, Ill,

[22] Filed: Dec. 29, 1969 [21] Appl.No.: 888,993

Louis T. Pirkey, John F. Lynch, Robert White and Jack C. Goldstein [S2] U.S.Cl..........

....166/l56, 166/291 ABSTRACT ....E21b 33/16 A dual-plug-cementing device has a first plug for traveling 166/290, 291, 153-156, down the operating string ahead of the cement which engages [51] Int.Cl.................. [58] FieldofSearch.........................

a liner wiper at the bottom of the operating string. Both wiper and plug in sealing engagement travel ahead of the cement to the cementing region. An elongated portion of the plug strikes a stop in the cementing region permitting the wiper to continue downward movement relative to the plug and break the seal permitting the cement to flow past the plug.

UNITED STATES PATENTS Brown................................166/291 X 3,223,170 12/1965 Mott 166/153X 48Claims, lobrawingFigures PATENTED u amz SHEET 1 [IF 4 FIG. I

M.Lebourg INVENTOR i BY flamed 120W,

ATTORNEYS 4 f 2,, x wmwwawuww m m m m PATENTED JAN 1 8 I972 sum 3 OF 4 FIG.4

M Lebourg INVENTOR BY M50! Arrow/ e a i PATENIEU JM I 8 B72 SHEET UF 4 FIG. 9

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wwgezzJUwuiee ATTORNEYS LINER-CEMENTING APPARATUS BACKGROUND OF THE INVENTION This invention relates generally to equipment for cementing liners in well bores and more particularly to well-bore-linercementing apparatus in which the interior surfaces of the operating string and the liner are wiped by means preceding the cement flow therethrough.

With respect to the field problems in the setting and cementing of liners in well bores, many of the failures occur as a result of the cementing operation, not the setting and hanging of the liner. According to the design of typical well-boresliner-cementing equipment now in use, a body of cement is introduced through an operating string into the liner itself and is then displaced into the annular space between the liner and the well bore, generally to a point inside a surrounding casing to which the upper end of the liner is usually anchored. A single wiper plug is ordinarily introduced into the liner immediately behind the cement in order to separate the latter from the displacing fluid and to wipe the cement from the interior surface of the liner as the cement is displaced therefrom.

It has previously been recognized to be desirable to introduce a wiper plug in front of the cement slug to separate it from contaminants since the well bore, liner, and perhaps operating string are ordinarily filled with well fluid and mud prior to the introduction of the cement. Where only one wiper plug is used, the head of the cement slug passing through the liner is obviously contaminated by the well fluid and mud. Moreover, ordinarily the cement channels through some of the mud in the liner leaving a large amount of contaminant in place, such that release of the wiper plug behind the cement contaminates the tail of the cement slug also. Thus, when the cement is finally placed, defects are likely to occur at the top and bottom of the liner.

Naturally, in such systems, it is necessary that some type of opening be provided at the lower end of the apparatus after the wiper plug preceding the cement lands, to allow the cement to pass from the lower end of the liner into the annular space between the liner and the well bore. US. Pat. No. 2,630,179 issued to C. C. Brown on Mar. 3, 1953, US. Pat. No. 3,223,170 issued to J. D. Mott on Dec. 14, 1965, US. Pat. No. 3,364,996, issued to C. C. Brown on Jan. 23, 1965, and an article in THE OIL AND GAS JOURNAL, Sept. 22, 1969, page 152 illustrate prior art well-bore-liner-cementing techniques and apparatus using a wiper plug preceding the cement. Some of these systems are not adaptable for wiping the operating string as well as the liner. Some systems utilize a frangible member which is broken by pressure, producing fragments which are likely to lodge in and disturb the operation of back pressure valves or ports. Other systems are limited by the lack of capacity to selectively control the displacement of the cement from the liner to the annular space between the liner and the well bore after the cement and wiper plug preceding it have reached the bottom of the liner. Still other systems require complex locking arrangements to ensure that the wiper plug preceding the cement remains in its lowermost position. Of course, additional locking devices present added sources of failure but moreover present greater difficulty in the further drilling operations often resumed after cementing because of the hardness of the materials from which such locking arrangements must generally be fabricated. Still further systems require that a resilient plug be extruded through an orifice which may not be possible if portions of rock of other contaminants lodge in the orifice.

Another problem frequently occurs due to the exothermic nature of the cement curing process which generates tremendous heat and accordingly significant temperatures and pressure increases. This pressure increase is often so great as to rupture or distort the liner itself.

Still another problem in this art is malfunction of the back pressure or back flow valve which may result from the contaminants ahead of the cement either lodging in or otherwise damaging the valve as they are displaced in front of the cement.

SUMMARY OF THE INVENTION Briefly, in accordance with this invention, these and other problems are alleviated by providing, in well bore liner cementing apparatus, a first plug means for wiping and sealing the operating string ahead of the cement, a first wiper means for wiping and sealing the liner ahead of the cement upon a predetermined engagement with said first plug, and a first stop means for displacing the first plug and wiper from the predetermined engagement thereby breaking the seal across the liner ahead of the cement. Preferably, the stop means effectively limits the downward movement of the first plug while permitting the first wiper to continue downward movement relevant thereto. A second plug means and wiper means may be similarly employed behind the cement, and there may be provided a second stop means for limiting the downward movement of the second plug means and wiper means but enabling slight upward movement for expansion of the cement. A back pressure of back flow valve assembly may be used to prevent flow of the contaminants ahead of the cement through the valve itself.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more particularly disclosed with reference to the specific embodiments disclosed in the drawings.

FIG. 1 is an elevation view, partly in section, of an embodiment according to the present invention showing the lower plug engaging the lower wiper;

FIG. 2 is a section of a portion of the apparatus taken along line 22 of FIG. 1;

FIG. 3 is an elevation view, partly in section, of the upper plug portion of the apparatus according to an embodiment of the invention;

FIGS 4 and 5 show the apparatus generally illustrated in FIG. 1 at different deployment positions;

FIG. 6 is an elevation view, generally in section, of a safety valve assembly particularly suitable for use with the apparatus shown in FIGS. 1, 4 and 5;

FIG. 7 is an elevation view in section of a particular feature of the preferred embodiment of the present invention;

FIG. 8 is an elevation view of an alternative device for use in lieu of the catching and anchoring lugs shown in FIG. ll;

FIG. 9 shows a further modification of the present invention; and

FIG. 10 shows a further modification of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings wherein like reference characters designate identical or corresponding parts throughout the several view, and more particularly to FIG. 1 thereof, the well-bore-liner-cementing apparatus of the present invention is shown inserted in a conventional well bore 10, the upper portion of which may or may not be lined with a casing 12 extending to the surface. As is also conventional, the apparatus includes a liner 14, which may be an elongate string of a plurality of interconnected tubular pipe sections sufficient to provide a liner of the desired length, and an operating string 16, which may similarly be an elongate string of a plurality of interconnected tubular pipe sections sufficient to provide an operating string of the desired length, at least partly disposed within the upper end of the liner. The liner may be set in the well bore and/or secured to the casing by an conventional means (not shown).

The cementing tool of the invention comprises a tubular mandrel disposed at the bottom of the operating string having two wipers 44 ad 64, and two plugs 74 and 1 l4 launched from the surface level ahead of and behind the cement slug, respectively.

Lower wiper 44 comprises a generally cylindrical elongate hollow sleeve 18, having an outside diameter slightly less than the inside diameter of operating string 16, sleeve 18 being releasably secured at its upper end to the lower end of the operating string by any convenient pressure-responsive means, such as shear pins 20. Sleeve 18, being at least partly disposed within operating string 16, forms an internal beveled shoulder 22 with respect to the interior surface of the operating string. A plurality of circumferentially spaced radial ports 23 are disposed at the lower end of sleeve 18.

The lower part of lower wiper 44 includes a generally cylindrical hollow end sleeve 24, having an inside diameter preferably substantially equal to that of sleeve 18, sleeve 24 being secured to the lower end of sleeve 18 by any conventional means, such as threaded connection 26, or it may be integral therewith. End sleeve 24 has disposed near the top thereof an internal threaded annular groove 28 and an external annular flange 30 with an annular recess or groove 32 in the upper surface thereof. The bottom of end sleeve 24 is provided with male threads 34.

Lower wiper 44 further comprises a generally cylindrical wiping element 36, having resilient wiping ribs 38, which wiping element circumscribes end sleeve 24, abuts against flange 30 thereof, and is secured in position by an externally grooved retaining ring 40 having female threads 42. Wiping element 36 may be made of rubber or some other resilient material having physical characteristics capable of performing the functions to be described. The outside diameter of wiping element 36, when it is not compressively biased inward, should be sufficiently larger than the inside diameter of liner 14 to wipe the interior surface of the latter when the former is moved downward therein. Sleeve 18, end sleeve 24, wiping element 36, and retaining ring 40 cooperate to form the first, or lower, wiper or wiping member 44.

Upper wiper 64 comprises a generally cylindrical hollow sleeve 46, having an inside diameter slightly greater than the outside diameter of operating string 16, sleeve 46 being releasably secured at its upper end to the lower end of the operating string by any conventional pressure-responsive means, such as shear pins 48. An internal flange 50 is provided at the lower end of sleeve 46.

A gasket seal 53 between wiper 64 and operating string 16 assures that wiper 64 is not released prematurely by pressure buildup in the region of gasket 53 caused by pressure leakage between sleeve 18 and operating string 16.

Below sleeve 46, a generally cylindrical wiping element 52, having resilient wiping ribs 54, circumscribes a generally cylindrical hollow sleeve 56 which is secured at its upper end to the lower end of sleeve 46 by any conventional means, such as threaded connection 58 and an external flange 60 at the bottom of sleeve 56. A washer 62 may be used between wiping element 52 and flange 60 and at other such bearing surfaces if desired. Wiping element 52 likewise wipes the interior surface of liner 14 when it moves down the liner. Sleeve 46, sleeve 56, and wiping element 52 cooperate to form the second, or upper, wiper or wiping member 64.

A plurality of catching lugs 66 and anchoring lugs 68 are secured to the lower end of upper wiper 64 by any conventional means. Catching lugs 66 are resiliently biased radially inward by appropriate means, such as spring portions 70; and anchoring lugs 68 are resiliently biased radially outward by appropriate means, such as spring portions 72. As can be seen in FIG. 1, the catching lugs and anchoring lugs extend into annular recess 32 and are restrained in their respective resiliently biased positions by lower wiper 44 prior to its deployment down the liner.

Of course, a variety of altemative devices may be used in lieu of catching lugs 66 and anchoring lugs 68. For example, as shown in FIG. 8, cast iron drag blocks, resiliently biased in appropriate recesses in sleeve 56 by compressed coil springs, may be urged against the lower wiper as at sleeve 18 for subsequent engagement with an appropriate recess in the upper plug. The drag block shown in FIG. 8 corresponds generally to catching lug 66 although it is evident that a similar drag block could also be used in lieu of anchoring lug 68.

A first, or lower, elongate generally cylindrical plug 74 is shown in FlG. l in a predetermined engagement with lower wiper 44. Lower plug 74 comprises a lower head portion 76, an intermediate portion 78, and an upper tail portion 80.

The head portion is a generally cylindrical elongate body having an internally threaded axial blind bore at its upper end and a tapered conically shaped lower end. An O-ring 82 forms an annular cement seal between the lower plug and lower wiper below radial ports 23. The conically shaped lower end is provided with a plurality of radial grooves or teeth. At the lower half of head portion, there are a plurality of longitudinally extending radial recesses 84 which may be more clearly understood by reference to FIG. 2 showing a cross section of the head portion taken along line 22.

Tail portion is generally cylindrically shaped and has an internally threaded axial blind bore at its lower end and a generally flat upper end. The tail portion has a body section 86 of an outside diameter slightly less than the inside diameter of sleeve 18 and an enlarged upper end section 88 of an outside diameter slightly less than the inside diameter of operating string 16 but greater than the inside diameter of sleeve 18, thereby forming a shoulder 00 which, upon engaging shoulder 22, prevents lower plug 74 from moving further downward relative to lower wiper 44.

Head portion 76 and tail portion 80 are interconnected by any convenient means, such as a stud 96 having male threads at each end forming intermediate section 78. The intermediate section further includes a plurality of annular wiping elements 98, having resilient wiping ribs 100, positioned around stud 96 and secured between the head and tail portions. Wiping elements 98 may be made of rubber or some other resilient material such that the wiping ribs are of suffrcient diameter as to form a cement seal across and wipe the interior surface of the operating string and then bend or compress enough to pass over shoulder 22 and enter sleeve 18.

After lower plug 74 is inserted into operating string 16 ahead of a slug of cement 101, the pressure on the cement is increased, driving the lower plug downward and thus wiping the interior surface of the operating string head of the cement by means os wiping elements 98. As the lower plug approaches the position shown in FIG. 1, an outwardly resiliently biased locknut 102 seated in an annular groove in head portion 76 approaches threaded annular groove 28. As locknut 102 will seat in groove 28 and further downward movement of lower plug 74 with respect to lower wiper 44 is effectively prevented by the bearing engagement of shoulder and sleeve 18. The locknut and threaded groove provide for selective displacement of the lower wiper and plug and from the predetermined engagement shown in FlG. l, as will become apparent.

Positioned at the bottom of liner l4 and secured thereto by any convenient means, such as threaded connection 104, is a generally cup-shaped stop member 106 having an axially extending generally cylindrical raised central portion 108 and a plurality of axial ports 110 passing through the bottom thereof. The upper surface of raised portion 108 is counterbored and provided with radial grooves or teeth so as to provide a mating surface with the conically shaped lower end of lower plug 74. The outside diameter of raised portion 108 may be the same as that of head portion 76 of lower plug 74, which is accordingly less than the inside diameter of lower wiper 44.

Referring now to FIG. 3, there is shown a head portion 112 of a second, or upper, plug 114. Head portion 112 is generally cylindrically shaped but has a longitudinally extending annular radial recess 116 near the bottom thereof. With the exception of the head portion the remainder of plug 114 may be essentially identical to lower plug 74.

The operation of the apparatus may be understood by reference to the drawings. With the parts of the apparatus arranged generally as shown in FIG. 1 but without lower plug 74, the apparatus will be run into well bore 10 through casing 12,

being supported for this movement by means of conventional derrick drawwork elements (not shown). When liner 14 has reached the point in the well bore at which it is to be ccmented, it is set therein, perhaps by anchoring the upper end to the casing by conventional means (not shown). Operating string 16, having lower and upper wipers 441 and 641 releasably attached thereto, may be lowered intothe well bore and the upper end of the liner in a separate operation, but in either event it is apparent that the operating string must extend'far enough into liner 14 to ensure that upper wiper 64 is disposed within the liner as shown in HO. 1.

Lower plug 74 is introduced into the upper end of the operating string ahead of cement 101, separating the cement from the previously introduced washing fluid and the well fluid and mud in advance thereof by means off wiping elements 98 which, with the lower plug itself, form a movable cement seal across the operating string. The introduction of additional cement causes increased pressure on lower plug 74, driving the lower plug downwardly through the operating string and wiping the interior surface of the latter ahead of the cement by means of wiping elements 98. When lower plug 74 is forced into a first position with respect to lower wiper 44 in the aforementioned predetermined engagement, as illustrated in FIG. 1, the pressure of the cement is increased still further, breaking shear pins 20, and releasing the lower wiper which will, together with the lower plug, travel downwardly through the liner, wiping the interior surface thereof ahead of the cement by means of wiping element 36. it should be apparent that, in the predetermined engagement lower wiper and plug 44 and 74 form a movable cement seal across the liner ahead of the cement since wiping element 36 sealingly engages liner Ml and wiping elements 98 sealingly engage the lower wiper at sleeve 18. Radial ports 23 are sealed by O-ring 82 positioned around the enlarged upper portion of head portion 76 of lower plug 74.

As is well known, the volume of cement to the introduced will have been calculated so that, when it has been fully displaced from the liner, it will have attained the desired height in the annular space between the liner and the well bore. After the calculated volume of cement has been introduced into the operating string, upper plug 114 illustrated in part in FIG. 2, will be introduced into the operating string. The upper plug similarly serves as a movable cement seal across the operating string but behind the cement and may be driven downwardly by increasing the pressure on the medium, usually salt water or well fluid, behind the upper plug. When the upper plug has been driven sufficiently far down the operating string,

v catching lug 66, previously released by the downward movement of the lower wiper, will engage annular recess 116 under the radially inward, resilient bias of spring portion 70 as seen in FIG. 3. Thereafter, the pressure is increased still further, breaking shear pins 48 and releasing the upper wiper which will, together with the upper plug, travel downwardly through the liner, wiping the interior surface thereof behind the cement of wiping element 52. lt should be apparent that upper wiper and plug 64 and 114, respectively, similarly form a movable cement seal across the liner but behind the cement.

Thus, as the pressure ton the upper plug is increased, the cement will be forced down the liner preceded by a movable cement seal formed by lower wiper and plug 44 and 74 and followed by a movable cement seal formed by upper wiper and plug 64 and 114. The cement moves in this fashion down the liner until the conically shaped lower end of lower plug 74 matingly engage the counterbored upper end of raised portion 108 of stop member 106 as shown in FIG. 4. The corresponding teeth on plug 74 and stop member 106 coengage one another to prevent relative radial movement thereby making subsequent drilling less difficult. The raised portion of the stop member effectively stops downward movement of the lower plug relative to the stop member. Downward movement of lower wiper 44 relative to lower plug 74 is selectively prevented by the engagement of threaded groove 28 with resilient locknut 102. However, as the pressure is increased,

the downward forces acting on the lower wiper will increase to the point where locknut 102 will disengage threaded groove 28. Thus, the predetermined engagement of the lower wiper and plug may be selectively displaced and the lower wiper may move to a second lower position in the liner as shown in FIG. 5. Teeth 34 on the lower surface of retaining ring 40 lock into mating recesses 107 or like mating teeth on stop member 106 to prevent relative rotation of these pieces during subsequent drill-out procedures. Of course, it should be apparent that the predetermined engagement may be displaced before the upper wiper and plug come into the operation if the lower wiper and plug engage the stop member before a sufficient volume of cement has been introduced.

As shown in FIG. 5, when the predetermined engagement of the lower wiper and plug has been displaced, the cement seal therebetween is broken and the cement may flow out of the liner into the annular space between the liner and the well bore. As illustrated, the cement flows through radial ports 23 at the bottom of sleeve 18, into the annular space between the sleeve and the reduced portion of the lower plug, into longitudinal recesses 84, and through ports 1 10 in stop member 106.

It should be apparent, however, that ports 23 may be eliminated if raised portion 108 of the stop member 106 is sufficiently long to ensure that lower wiper 44 is displaced far enough relative to lower plug 74 so that the upper end of sleeve 18 is below the lowermost wiping element 98. In that case, cement would enter the annular space between the lower wiper and plug at the top of sleeve 18, there being no seal between wiping element 98 and the sleeve. Also, if the selective disposition of cement out of the liner were not required, locknut 102 could be eliminated.

When the pressure acting on the upper wiper and plug has forced them down the liner as far as is desired to displace the appropriate volume of cement from the liner, anchoring lug 68 will engage on annular groove (not shown) in the inside of the liner to prevent upward movement of the upper wiper and plug after the downward pressure thereon is released. With the apparatus so disposed, the cement is allowed to harden and effect a secure bond between the liner and the well bore.

it may be desirable and often is necessary to use a back pressure or back flow safety valve assembly to positively prevent flow of cement, once displaced, back into the liner should the pressure in the well bore suddenly increase or the displacing pressure suddenly decrease. Such a valve is also useful to permit the cement to harden while not maintaining pressure on the hole. Accordingly, such a valve assembly 1118, especially suitable for use with the previously described apparatus, is shown in FIG. 6.

Cup-shaped stop member 106 comprises a generally cylindrical sleeve internally threaded at its upper end for mating engagement with external threads on the lower end of liner 14. Sleeve 120 has a radially extending internal flange 122 and a small cylindrical sleeve 124 integrally secured to flange 122 and positioned coaxially within sleeve 120. Flange 122 and sleeve 124 have a coaxial longitudinal throughbore 126 and a plurality of radial ports 128 fluidly communicating with bore 126. Generally, cylindrical raised portion 108 is slidably positioned in bore 126 and has a coaxial longitudinal throughbore 130 and a plurality of radial ports 132 and 134 in fluid communication with bore 130 positioned in the intermediate and lower sections, respectively. A spring-loaded valve 136 is conventionally secured to sleeve 124 at the lower end thereof and selectively closes bores 126. A plurality of annular sealing means, such as O-rings 138, are disposed around raised portion 108 at longitudinally spaced points thereon.

With valve assembly 118 in the position illustrated in H6. 6, the two lower O-rings provide a cement seal between raised portion 108 above and below port 13 1 and flange 122 and sleeve 124 above and below port 128; and ports 128 and 134 are aligned for fluid communication therethrough. With the assembly as shown in FIG. 6, the contaminants being wiped by lower wiper 44 and plug 74 ahead of the cement pass through either the open top of raised portion 108 or port 132, bore 130, ports 134, and ports 128 to the annular space between the liner and the well bore without passing through valve 136. When lower plug 74 reaches the end of its path and bears against raised portion 108, the raised portion slides downward in bore 126, and the two upper O-rings provide a cement seal between raised portion 108 and flange 122 and sleeve 124 above and below port 128 thereby closing port 128 to fluid flow. Thereafter, when lower wiper 44 and plug 74 are displaced from the predetermined engagement, cement flows through port 132, bore 130, bore 126, and valve 136 into the annular space between the liner and the well bore. Of course, shear rings 123 or pins may be positioned between raised portion 108 and flange 122 to provide the valve assembly with selective displacement capability. It is apparent that the contaminants ahead of the cement cannot adversely effect the operation of valve 136 in valve assembly 118 since the contaminants do not flow through the valve but instead bypass it. That it, the valve is used to allow one-way flow of cement only, not the contaminants. Thus, the valve assembly of the present invention in cooperation with the remainder of the apparatus minimizes the opportunity for malfunctions in the back pressure or back flow valve. Of course, other less sophisticated means of preventing back flow may also be used.

The exothermic nature of the cement curing process may cause a significant increase in temperature in the cementing region which accordingly may result in significant pressure increases. To relieve this pressure increase, it may be desirable to use a generally cylindrical sliding sealing sleeve 140 as shown in FIG. 7. Instead of merely an internal annular groove at the bottom of liner 14 as heretofore discussed, the liner may be provided with an internal longitudinally extending elongate radial annular recess 142. Sleeve 140 may be initially slidably positioned within recess 142 at the lower end thereof, and an annular sealing means, such as O-ring 144, may be interposed between sleeve 140 and liner 14 to effect a seal therebetween.

As upper wiper 64 approaches the end of this downward movement, washer 62, or some other appropriate stop lug radially extending from sleeve 56, will engage the upper end of sleeve 140 positioned at the lower end of recess 142 either initially or driven thereto by the upper wiper. When the upper wiper engages the upper end of sleeve 140 to prevent movement downward relative thereto, anchoring lug 68 engages the lower end of sleeve 140 to prevent movement upward relative thereto. Then, if pressure of the cement increases, sleeve 140, with upper wiper 64 secured thereto, slides upward in recess 142 to permit suitable expansion. Also, to improve the drillability of the apparatus after the cement has cured, antirotation clutch pins or lugs (not shown) may be advantageously used to prevent rotation between sleeve 140 and the liner.

With reference to FIG. 9, there is shown a further modification of the invention. In cases where the plugs are too small to provide good wiping action in the operating string, an auxiliary plug having a wider diameter may be releasably secured to the upper end of the plug. For example, the operating string may have a wider diameter near the surface than down hole or the operating string may be of a significantly larger diameter than the setting tool; in either case the usual plug may not be able to satisfactorily wipe the wider diameter portion. Thus, the auxiliary plug 113 will wipe the wider diameter portion; and, upon reaching the reduced diameter portion, the pressure may be increased to shear the upper portion of the rubber auxiliary wiper cup thereby releasing the plug for further downward motion through the reduced diameter portion. Of course, the auxiliary plug may be releasably secured to the plug by the shear membrane 115, as illustrated, or by other suitable techniques.

A further modification of the present invention is sown in FIG. wherein a sealing means 144 is disposed above upper wiper 64 between operating string 16 and liner 14 to prevent flow upward therebetween during the lowering of the cementing apparatus into the borehole and during the cement flow prior to release of the upper wiper. Due to the illustrated incline of the wiping ribs 54 ofwiping element 52, well fluid or cement below wiping element 52 may readily flow upward past the wiping element if the differential pressure on each side of the element is of sufficient magnitude in the upward direction. During the aforementioned operations quite significant pressure surges below the wiper element are encountered, and sealing means 144 may be quite useful.

However, the upward oriented differential pressure causing well fluid or cement to flow upward past wiping element 52 is in the form of pressure surges, not content high pressure. Thus, well fluid or cement trapped above the wiping element may reach a significantly high downward oriented differential pressure as the pressure surge below the wiping element diminishes unless some other pressure release means is provided. If that downward oriented differential pressure were of sufficient magnitude, upper wiper 64 would prematurely shear away from operating string 16 to relieve the pressure. Thus, there is provided a pressure release means, such as perforated sleeve 146 secured within liner 14 to operating string 16 by any conventional means. Perforated sleeve 146 extends at least now enough to keep wiping element 52 from sealingly engaging the liner. The perforated sleeve provides fluid communication inside liner 14 above and below wiping element 52 to ensure against premature release of the upper wiper. Gasket seal 53 functions as described in FIG. 1. Additionally, an O-ring seal 55 between wiper 64 and operating string 16 ensures that the pressure seal between the wiper and the operating string will be retained until ribs 54 engage liner 14.

Obviously, numerous modifications and variations of the present invention are possible in the light of the above teachings. For example, the valve assembly may be constructed so as to positively close by providing a closed upper end of raised portion 108 and lengthening sleeve 124 to provide a longer space above valve 136 into which piece 108 may slide. The interior of the sleeve 124 is provided with a selectively releasable stop means such as a shearable ring 123 at a position so that ports 132 are held above flange 122. Upon the first plugs striking piece 108, it would move down to the stop means and cement and flow through ports 132 and valve 136. If no stop means are provided for the upper plug, the upper plug could continue down the liner until it stuck the lower plug, and by increasing the pressure further, the stop means would be released and raised portion 108 would be moved downward until radial ports 132 were below flange 122. At that point, appropriate conventional locking means might secure raised portion 108 against upward movement, thus providing positive closure of the valve assembly. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

What is claimed is:

1. Apparatus for cementing a liner in a well bore wherein the cement enters the liner through an operating string and flows downwardly through the liner, comprising:

first plug means for effecting a movable cement seal across the operating string ahead of the cement and for wiping the interior surface of the operating string ahead of the cement;

first wiper means for effecting a movable cement seal across the liner ahead of the cement upon engagement of said first plug for movement therewith down the liner and for wiping the interior surface of the liner ahead of the cement; and

first stop means for displacing said first plug means and said first wiper means from said engagement to break said movable cement seal across the liner.

2. The apparatus of claim 1 further comprising means for selectively securing said first plug means and first wiper means in said engagement and for selectively releasing said first plug means and first wiper means from said engagement.

3. The apparatus of claim 1 wherein said first wiper means has a central space therethrough and said first plug means occupies at least a portion of said annular space when said first plug means and said first wiper means are in said engagement.

4. The apparatus of claim 1 further comprising means releasably secured to said first plug means for effecting a movable cement seal across and for wiping the interior surface of a pipe string of greater diameter than the operating string to the lower end of which the upper end of the operating string is secured.

5. The apparatus of claim 1 further comprising means for preventing cement flow back into the liner from between the liner and the well bore.

6. The apparatus of claim 5 further comprising valve means defining a first fluid path from said liner past said stop means for fluid preceding said movable cement seal, said valve means being actuable in response to the displacement of said first plug means and said first wiper means to direct the cement past said stop means by a second fluid path different from the said first fluid path of the fluid preceding said movable cement seal.

7. The apparatus of claim 1 further comprising first wiper means angular stop means for preventing rotational movement of said first wiper means after disengagement from said first plug means.

8. The apparatus of claim 7 wherein said first wiper means angular stop means comprises mating teeth on said first wiper means and said first stop means.

9. The apparatus of claim 1 further comprising:

second plug means for effecting a movable cement seal across the operating string behind the cement and for wiping the interior surface of the operating string behind the cement;

second wiper means for effecting a movable cement seal across the liner behind the cement upon engagement of said second plug means for movement therewith down the liner and for wiping the interior surface of the liner behind the cement; and

second stop means for stopping the movement of said second plug means and second wiper means down the liner.

10. The apparatus of claim 9 further comprising means for enabling upward movement of said second plug means and second wiper means in response to the expansion of the cement during the cure thereof.

11. The apparatus of claim 1 wherein said first stop means comprises means for stopping the movement of said first plug means down the liner relative to said first wiper means.

12. The apparatus of claim 11 wherein said first stop means further comprises surface means for contacting said first plug means and arresting the downward motion thereof prior to said first wiper means contacting said first stop means to produce movement of said first plug means upward relative to said first wiper means.

13. The apparatus of claim 12 further comprising first plug means angular stop means for preventing rotational movement of said first plug means after contacting said surface means of said first stop means.

14. The apparatus of claim 13 wherein said first plug means angular stop means comprises mating teeth on said first plug means and said surface means of said first stop means.

15. Apparatus for cementing a liner in a well bore wherein the cement enters the liner through an operating string and flows downwardly through the liner to the cementing region,

Comprising:

lower wiper means adapted to move down the liner for wiping the interior surface thereof ahead of cement, said lower wiper means being releasably secured at the proximate end of the operating string and having a central longitudinal passageway therethrough; lower plug means insertable in the operating string at the surface and adapted to move down the operating string for effecting a movable cement seal thereacross and for wiping the interior surface thereof ahead of the cement, said lower plug means including means for engaging said lower wiper means for conjoint movement down the liner with said lower plug means in a first position relative to said lower wiper means and means for effecting a cent seal across said longitudinal passageway and in conjunction with said lower wiper means for effecting a movable cement seal across the liner ahead of the cement when said lower plug means and said lower wiper means are in said first position; and

lower longitudinal stop means in the cementing region for moving said lower plug means to a second position relative to said lower wiper means thereby establishing a cement path past said lower plug means and said lower wiper means and enabling cement flow from the liner to the annular space between the liner and the wall of the well bore. I 16. The apparatus of claim 15 further comprising means for selectively securing said lower plug means and said lower wiper means in said first position and for selectively releasing said lower plug means and said lower wiper means for movement or said second position.

17. The apparatus of claim 15 wherein at least a portion of said lower plug means occupies at least a portion of said longitudinal passageway when said lower plug means and said lower wiper means are in said first position.

18. The apparatus of claim 15 further comprising means releasably secured to said lower plug means for effecting a movable cement seal across and for wiping the interior surface of a pipe string of greater diameter than the operating string to the lower end of which the upper end of the operating string is secured.

19. The apparatus of claim 15 further comprising means for preventing cement back flow into the liner from between the liner and the well bore.

20. The apparatus of claim 19 wherein said means for preventing cement back flow into the liner comprises:

back flow valve means for enabling one-way flow out of the liner and for preventing flow back into the liner from between the liner and the well bore; and

valve assembly means for directing the flow ahead of the cement around said valve means and in response to said lower plug means for directing the entire flow of the ocment through said valve means.

21. The apparatus of claim 15 further comprising lower wiper angular stop means for preventing rotational movement of said lower wiper means in said second position.

22. The apparatus of claim 21 wherein said lower wiper means angular stop means comprises mating teeth on said lower wiper means and said lower longitudinal stop means.

23. The apparatus of claim 15 wherein said lower longitudinal stop means comprises means for stopping the movement of said lower plug means down the liner relative to'said lower wiper means.

24. The apparatus of claim 23 wherein said lower longitudinal stop means further comprises surface means for contacting said lower plug means and arresting the downward motion thereof prior to said lower wiper means contacting said longitudinal stop means to produce movement of said lower plug means upward relative to said lower wiper means.

25. The apparatus of claim 24 further comprising lower plug means angular stop means for preventing rotational movement of said lower plug r neans after contacting said surface means of said lower longitudinal stop means.

26. The apparatus of claim 25 wherein said lower plug means angular stop means comprises mating teeth on said lower plug means and said surface means of said lower longitudinal stop means.

27. The apparatus of claim 15 further comprising:

upper wiper means adapted to move down the liner for wiping the interior surface thereof behind the cement, said upper wiper means being releasably secured at the proximate end of the operating string and having a central longitudinal passageway therethrough;

' upper plug means insertable in the operating string at the surface and adapted to move down the operating string for effecting a movable cement seal thereacross and for wiping the interior surface thereof behind the cement, said upper plug means including means for engaging said upper wiper means for conjoint movement down the liner and means for effecting a cement seal across said longitudinal passageway and in conjunction with said upper wiper means for effecting a movable cement seal across the liner behind the cement; and

upper longitudinal stop means for stopping the movement of said upper plug means and said upper wiper means down the liner. 28. The apparatus of claim 27 further comprising means for enabling upward movement of said upper plug means and said upper wiper means in response to the expansion of the cement during the cure thereof.

29. The apparatus of claim 27 including a pressure equalization port permitting fluid communication between the fluid in the operating string and fluid within the annular volume between the operating string and the liner to prevent pressure buildup in said annular volume and prevent premature release of said lower wiper means.

30. The apparatus of of claim 27 further comprising; fluid seal means disposed between said operating string and said liner above said upper wiper means to prevent upward flow between the operating string and the liner; and

means to equalize the pressure withing the annular volume defined by the operating string and the liner between said lower wiper means and said fluid seal means with the pressure within said operating string to prevent pressure buildup within said annular volume from prematurely disconnecting said wiper means from said operating string.

31. The apparatus of claim 30 wherein said means to equalize pressure comprises:

holding means to prevent the wiper blades of said upper wiper means from forming a fluid seal with the liner; and port means to enable pressure equalization between said annular volume and the interior of said operating string.

32. The apparatus of claim 31 wherein said holding means is a sleeve affixed to the operating string and is adapted to compress to said upper wiper to hold said wiper out of contact with the liner, and wherein said sleeve has pressure equalization ports to prevent pressure buildup between said upper wiper means and said sleeve.

33. Apparatus for cementing a liner a well bore wherein the cement enters the liner through an operating string and flows downwardly through the liner, comprising:

lower wiper means for movement down the liner ahead of the cement, said lower wiper means including a first generally cylindrical sleeve having a longitudinal generally circular passageway therethrough and annular wiping means sealingly attached around said first sleeve for movement therewith and sealingly engaging the interior surface of the liner in at least one closed line of contact to wipe the interior surface of the liner ahead of the cement;

first releasable attachment means for releasably attaching said lower wiper means to the end of the operating string;

lower plug means adapted to be inserted in the operating string at the surface for movement down the operating string ahead of the cement and down the liner with said lower wiper means ahead of the cement, said lower plug means including an elongate generally cylindrical head portion insertable in the longitudinal passageway in said first sleeve,

engaging means for releasably engaging said lower wiper means in a first position relative to said lower plug means, for releasing said lower wiper means from said operating string, and for pushing said lower wiper means down the liner ahead of the cement in the first position,

sealing and wiping means for effecting a movable cement seal across the operating string, for wiping the interior surface of the operating string ahead of the cement, and for effecting a cement seal across the longitudinal passageway in said first sleeve when said lower plug means and said lower wiper means are in said first position;

longitudinal stop means for stopping the downward movement of said lower plug means and for permitting said lower wiper means to move down the liner to a second position relative to said lower plug means lower than said first position, thereby breaking the seal between said lower plug means and said lower wiper means.

34. The apparatus of claim 33 wherein said first sleeve is provided with at least one radial passageway therethrough above said annular wiping means, and wherein said head portion of said lower plug means comprises:

a lower section having at least one longitudinally extending radial channel longitudinally aligned with the radial passageway in said first sleeve when and said lower wiper means in said second position relative to said lower plug means.

35. The apparatus of claim 33 wherein at least a portion of said first sleeve is positioned within the operating string, and wherein said engaging means comprises:

a generally cylindrical tail portion, a section of which having an outside diameter smaller than the inside of the operating string but larger than the inside diameter of the longitudinal passageway in said first sleeve, whereby said tail portion any abut and longitudinally bear against said first sleeve.

36. The apparatus of claim 33 further comprising means for preventing cement back flow into the liner from between the liner and the well bore.

37 The apparatus of claim 36 wherein said means for preventing cement back flow into the liner from between the liner and the well bore.

back flow valve means for enabling one-way, flow out of the liner and for preventing flow back into the liner from between the liner and the well bore; and

valve assembly means for directing the flow ahead of the cement around said valve means and in response to said lower plug means for directing the entire flow to the cement through said valve means.

38. The apparatus of claim 33 further comprising:

upper wiper means for movement down the liner behind the cement, said upper wiper means including a second generally cylindrical sleeve having a generally circular longitudinal passageway therethrough,

catching means resiliently biased radially inward with respect to said longitudinal passageway and extending therein, and

annular wiping means sealingly attached around said second sleeve for movement therewith and sealingly engaging the interior surface of the liner in at least one closed line of contact to wipe the interior surface of the liner behind the cement;

second releasable attachment means for releasably attaching said upper wiper means to the operating string;

upper plug means for movement down the operating string behind the cement and down the liner with said upper wiper behind the cement, said upper plug means including a generally cylindrical head portion having a circumferential radial groove therein for reception of said catching means and means for effecting a movable cement seal across the liner behind of the cement in conjunction with said upper wiper means when engaged therewith.

39. The apparatus of claim 38 further comprising means for stopping the downward movement of said upper wiper means and said upper plug means.

40. The apparatus of claim 39 wherein said means for stopping is a floating means for enabling upward movement of said second plug means and second wiper means in response to the expansion of the cement during the cure thereof.

41. The apparatus of claim 38 including means for preventing cement flow back into said liner from between the liner and the well bore.

42. The apparatus of claim 41 wherein said longitudinal stop means and said means for preventing cement flow are incorporated into a single assembly positioned in the cementing zone and comprising a stop sleeve disposed at the end of said liner in the cementing zone;

a back flow valve disposed in the lower portion of said stop sleeve;

slidable flow-directing means disposed in said stop sleeve in a first position to establish fluid flow path past said stop sleeve and around said back flow valve, said slidable flow directing means being progressively movable to a second position to establish a cement flow path past said stop sleeve and through said back flow valve and to a third position to seal said stop sleeve against any flow.

43. The apparatus of claim 42 wherein said slidable flow directing means comprises an elongated tubular member closed at one end and having at least two ports longitudinally disposed thereon.

44. The apparatus of claim 42 including selectively releasable means to maintain said slidable flow-directing means in said second position.

45. The apparatus of claim 44 wherein said selectively releasable means is a pressure releasable means.

46. In a dual-plug-cementing apparatus wherein two annular liner wiper means are disposed in a liner at the end of an operating string and wherein said two wipers are adapted to be successively engaged and released from the operating string by two plug means to form movable cement seals across the liner in front of and behind the cement, the improvement comprising:

fluid seal means disposed between the operating string and the liner above the upper of said two wiper means to prevent upward flow past said fluid seal means between the operating string and the liner, and means to equalize the pressure within the annular volume defined by the operating string and the liner between the lower of said two wiper means and said fluid seal means with the pressure within the operating string to prevent pressure buildup within said annular volume from prematurely releasing said wiper means. 47. The apparatus of claim 46 wherein said means to equalize pressure comprises:

holding means to prevent the wiper blades of said upper wiper means from forming a fluid seal with the liner; and port means to enable pressure equalization between said annular volume and the interior of said operating string. 48. The apparatus of claim 47 wherein said holding means is a sleeve affixed to the operating string and is adapted to compress the said upper wiper to hold said wiper out of contact with the liner, and wherein said sleeve has pressure equaliza tion ports to prevent pressure buildup between said upper wiper means and said sleeve.

Patent No. 3, 35,2 Dated January 18, 1972 Inventor(s) Maurice P. Lebourg It is certified that error appears inthe above-identified patent and that said Letters Patent are hereby corrected as shown below: [Please disregard the numbers in the margin of the patent and count down from the top of each column.]

Column 2, line 73, "ad" should be --and--.

Column 3, line 50, after "connection 58" insert --at flange 50.

Wiping element 52 is secured between flange 50.

Column 4, line ll, "head" should be --ahead-;

line M2, "os" should be --of--.

Column 5, line 58, change "ton" to --on--.

Column 7, line 36, change "this" to its--; line 67, "sown" should be --shown-.

Column 8, line 20, "now" should be --low--;

line 38, after "cement", change "and" to --would--; third line of claim 3, "annular" should be --central-.

Column 10, line 1, change "cent" to --cement--;

fifth line of claim 16, change "or" to --to-; second line of claim 21, after "wiper" insert --means--.

Column ll, first line of claim 33, after "liner" insert --in-.

Column 12, seventh line of claim 3 4, delete "and";

eighth line of claim 3 .after "means" insert --is--; eighth line of claim 35, change "any" to --may--; second and third lines of claim 37, delete "from between the liner and the well bore." and substitute therefor -comprises:-; I ninth line of claim 37, change "to" to -of--.

Signed and sealed this 5th day of September 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

1. Apparatus for cementing a liner in a well bore wherein the cement enters the liner through an operating string and flows downwardly through the liner, comprising: first plug means for effecting a movable cement seal across the operating string ahead of the cement and for wiping the interior surface of the operating string ahead of the cement; first wiper means for effecting a movable cement seal across the liner ahead of the cement upon engagement of said first plug for movement therewith down the liner and for wiping the interior surface of the liner ahead of the cement; and first stop means for displacing said first plug means and said first wiper means from said engagement to break said movable cement seal across the liner.
 2. The apparatus of claim 1 further comprising means for selectively securing said first plug means and first wiper means in said engagement and for selectively releasing said first plug means and first wiper means from said engagement.
 3. The apparatus of claim 1 wherein said first wiper means has a central space therethrough and said first plug means occupies at least a portion of said annular space when said first plug means and said first wiper means are in said engagement.
 4. The apparatus of claim 1 further comprising means releasably secured to said first plug means for effecting a movable cement seal across and for wiping the interior surface of a pipe string of greater diameter than the operating string to the lower end of which the upper end of the operating string is secured.
 5. The apparatus of claim 1 further comprising means for preventing cement flow back into the liner from between the liner and the well bore.
 6. The apparatus of claim 5 further comprising valve means defining a first fluid path from said liner past said stop means for fluid preceding said movable cement seal, said valve means being actuable in response to the displacement of said first plug means and said first wiper means to direct the cement past said stop means by a second fluid path different from the said first fluid path of the fluid preceding said movable cement seal.
 7. The apparatus of claim 1 further comprising first wiper means angular stop means for preventing rotational movement of said first wiper means after disengagement from said first plug means.
 8. The apparatus of claim 7 wherein said first wiper means angular stop means comprises mating teeth on said first wiper means and said first stop means.
 9. The apparatus of claim 1 further comprising: second plug means for effecTing a movable cement seal across the operating string behind the cement and for wiping the interior surface of the operating string behind the cement; second wiper means for effecting a movable cement seal across the liner behind the cement upon engagement of said second plug means for movement therewith down the liner and for wiping the interior surface of the liner behind the cement; and second stop means for stopping the movement of said second plug means and second wiper means down the liner.
 10. The apparatus of claim 9 further comprising means for enabling upward movement of said second plug means and second wiper means in response to the expansion of the cement during the cure thereof.
 11. The apparatus of claim 1 wherein said first stop means comprises means for stopping the movement of said first plug means down the liner relative to said first wiper means.
 12. The apparatus of claim 11 wherein said first stop means further comprises surface means for contacting said first plug means and arresting the downward motion thereof prior to said first wiper means contacting said first stop means to produce movement of said first plug means upward relative to said first wiper means.
 13. The apparatus of claim 12 further comprising first plug means angular stop means for preventing rotational movement of said first plug means after contacting said surface means of said first stop means.
 14. The apparatus of claim 13 wherein said first plug means angular stop means comprises mating teeth on said first plug means and said surface means of said first stop means.
 15. Apparatus for cementing a liner in a well bore wherein the cement enters the liner through an operating string and flows downwardly through the liner to the cementing region, comprising: lower wiper means adapted to move down the liner for wiping the interior surface thereof ahead of cement, said lower wiper means being releasably secured at the proximate end of the operating string and having a central longitudinal passageway therethrough; lower plug means insertable in the operating string at the surface and adapted to move down the operating string for effecting a movable cement seal thereacross and for wiping the interior surface thereof ahead of the cement, said lower plug means including means for engaging said lower wiper means for conjoint movement down the liner with said lower plug means in a first position relative to said lower wiper means and means for effecting a cent seal across said longitudinal passageway and in conjunction with said lower wiper means for effecting a movable cement seal across the liner ahead of the cement when said lower plug means and said lower wiper means are in said first position; and lower longitudinal stop means in the cementing region for moving said lower plug means to a second position relative to said lower wiper means thereby establishing a cement path past said lower plug means and said lower wiper means and enabling cement flow from the liner to the annular space between the liner and the wall of the well bore.
 16. The apparatus of claim 15 further comprising means for selectively securing said lower plug means and said lower wiper means in said first position and for selectively releasing said lower plug means and said lower wiper means for movement or said second position.
 17. The apparatus of claim 15 wherein at least a portion of said lower plug means occupies at least a portion of said longitudinal passageway when said lower plug means and said lower wiper means are in said first position.
 18. The apparatus of claim 15 further comprising means releasably secured to said lower plug means for effecting a movable cement seal across and for wiping the interior surface of a pipe string of greater diameter than the operating string to the lower end of which the upper end of the operating string is secured.
 19. The apparatus of claim 15 further comprising means for preventing cement back flow into the liner from between the liner and the well bore.
 20. The apparatus of claim 19 wherein said means for preventing cement back flow into the liner comprises: back flow valve means for enabling one-way flow out of the liner and for preventing flow back into the liner from between the liner and the well bore; and valve assembly means for directing the flow ahead of the cement around said valve means and in response to said lower plug means for directing the entire flow of the cement through said valve means.
 21. The apparatus of claim 15 further comprising lower wiper angular stop means for preventing rotational movement of said lower wiper means in said second position.
 22. The apparatus of claim 21 wherein said lower wiper means angular stop means comprises mating teeth on said lower wiper means and said lower longitudinal stop means.
 23. The apparatus of claim 15 wherein said lower longitudinal stop means comprises means for stopping the movement of said lower plug means down the liner relative to said lower wiper means.
 24. The apparatus of claim 23 wherein said lower longitudinal stop means further comprises surface means for contacting said lower plug means and arresting the downward motion thereof prior to said lower wiper means contacting said longitudinal stop means to produce movement of said lower plug means upward relative to said lower wiper means.
 25. The apparatus of claim 24 further comprising lower plug means angular stop means for preventing rotational movement of said lower plug means after contacting said surface means of said lower longitudinal stop means.
 26. The apparatus of claim 25 wherein said lower plug means angular stop means comprises mating teeth on said lower plug means and said surface means of said lower longitudinal stop means.
 27. The apparatus of claim 15 further comprising: upper wiper means adapted to move down the liner for wiping the interior surface thereof behind the cement, said upper wiper means being releasably secured at the proximate end of the operating string and having a central longitudinal passageway therethrough; upper plug means insertable in the operating string at the surface and adapted to move down the operating string for effecting a movable cement seal thereacross and for wiping the interior surface thereof behind the cement, said upper plug means including means for engaging said upper wiper means for conjoint movement down the liner and means for effecting a cement seal across said longitudinal passageway and in conjunction with said upper wiper means for effecting a movable cement seal across the liner behind the cement; and upper longitudinal stop means for stopping the movement of said upper plug means and said upper wiper means down the liner.
 28. The apparatus of claim 27 further comprising means for enabling upward movement of said upper plug means and said upper wiper means in response to the expansion of the cement during the cure thereof.
 29. The apparatus of claim 27 including a pressure equalization port permitting fluid communication between the fluid in the operating string and fluid within the annular volume between the operating string and the liner to prevent pressure buildup in said annular volume and prevent premature release of said lower wiper means.
 30. The apparatus of of claim 27 further comprising; fluid seal means disposed between said operating string and said liner above said upper wiper means to prevent upward flow between the operating string and the liner; and means to equalize the pressure withing the annular volume defined by the operating string and the liner between said lower wiper means and said fluid seal means with the pressure within said operating string to prevent pressure buildup within said annular volume from prematurely disconnecting said wiper means from said operating string.
 31. The apparatus of claim 30 wherein said means to equalize pressure comprises: holding means To prevent the wiper blades of said upper wiper means from forming a fluid seal with the liner; and port means to enable pressure equalization between said annular volume and the interior of said operating string.
 32. The apparatus of claim 31 wherein said holding means is a sleeve affixed to the operating string and is adapted to compress to said upper wiper to hold said wiper out of contact with the liner, and wherein said sleeve has pressure equalization ports to prevent pressure buildup between said upper wiper means and said sleeve.
 33. Apparatus for cementing a liner a well bore wherein the cement enters the liner through an operating string and flows downwardly through the liner, comprising: lower wiper means for movement down the liner ahead of the cement, said lower wiper means including a first generally cylindrical sleeve having a longitudinal generally circular passageway therethrough and annular wiping means sealingly attached around said first sleeve for movement therewith and sealingly engaging the interior surface of the liner in at least one closed line of contact to wipe the interior surface of the liner ahead of the cement; first releasable attachment means for releasably attaching said lower wiper means to the end of the operating string; lower plug means adapted to be inserted in the operating string at the surface for movement down the operating string ahead of the cement and down the liner with said lower wiper means ahead of the cement, said lower plug means including an elongate generally cylindrical head portion insertable in the longitudinal passageway in said first sleeve, engaging means for releasably engaging said lower wiper means in a first position relative to said lower plug means, for releasing said lower wiper means from said operating string, and for pushing said lower wiper means down the liner ahead of the cement in the first position, sealing and wiping means for effecting a movable cement seal across the operating string, for wiping the interior surface of the operating string ahead of the cement, and for effecting a cement seal across the longitudinal passageway in said first sleeve when said lower plug means and said lower wiper means are in said first position; longitudinal stop means for stopping the downward movement of said lower plug means and for permitting said lower wiper means to move down the liner to a second position relative to said lower plug means lower than said first position, thereby breaking the seal between said lower plug means and said lower wiper means.
 34. The apparatus of claim 33 wherein said first sleeve is provided with at least one radial passageway therethrough above said annular wiping means, and wherein said head portion of said lower plug means comprises: a lower section having at least one longitudinally extending radial channel longitudinally aligned with the radial passageway in said first sleeve when and said lower wiper means in said second position relative to said lower plug means.
 35. The apparatus of claim 33 wherein at least a portion of said first sleeve is positioned within the operating string, and wherein said engaging means comprises: a generally cylindrical tail portion, a section of which having an outside diameter smaller than the inside of the operating string but larger than the inside diameter of the longitudinal passageway in said first sleeve, whereby said tail portion any abut and longitudinally bear against said first sleeve.
 36. The apparatus of claim 33 further comprising means for preventing cement back flow into the liner from between the liner and the well bore. 37 The apparatus of claim 36 wherein said means for preventing cement back flow into the liner from between the liner and the well bore. back flow valve means for enabling one-way, flow out of the liner and for preventing flow back into the liner from between the liner and the well bore; and valve assembly means for directing The flow ahead of the cement around said valve means and in response to said lower plug means for directing the entire flow to the cement through said valve means.
 38. The apparatus of claim 33 further comprising: upper wiper means for movement down the liner behind the cement, said upper wiper means including a second generally cylindrical sleeve having a generally circular longitudinal passageway therethrough, catching means resiliently biased radially inward with respect to said longitudinal passageway and extending therein, and annular wiping means sealingly attached around said second sleeve for movement therewith and sealingly engaging the interior surface of the liner in at least one closed line of contact to wipe the interior surface of the liner behind the cement; second releasable attachment means for releasably attaching said upper wiper means to the operating string; upper plug means for movement down the operating string behind the cement and down the liner with said upper wiper behind the cement, said upper plug means including a generally cylindrical head portion having a circumferential radial groove therein for reception of said catching means and means for effecting a movable cement seal across the liner behind of the cement in conjunction with said upper wiper means when engaged therewith.
 39. The apparatus of claim 38 further comprising means for stopping the downward movement of said upper wiper means and said upper plug means.
 40. The apparatus of claim 39 wherein said means for stopping is a floating means for enabling upward movement of said second plug means and second wiper means in response to the expansion of the cement during the cure thereof.
 41. The apparatus of claim 38 including means for preventing cement flow back into said liner from between the liner and the well bore.
 42. The apparatus of claim 41 wherein said longitudinal stop means and said means for preventing cement flow are incorporated into a single assembly positioned in the cementing zone and comprising a stop sleeve disposed at the end of said liner in the cementing zone; a back flow valve disposed in the lower portion of said stop sleeve; slidable flow-directing means disposed in said stop sleeve in a first position to establish fluid flow path past said stop sleeve and around said back flow valve, said slidable flow directing means being progressively movable to a second position to establish a cement flow path past said stop sleeve and through said back flow valve and to a third position to seal said stop sleeve against any flow.
 43. The apparatus of claim 42 wherein said slidable flow directing means comprises an elongated tubular member closed at one end and having at least two ports longitudinally disposed thereon.
 44. The apparatus of claim 42 including selectively releasable means to maintain said slidable flow-directing means in said second position.
 45. The apparatus of claim 44 wherein said selectively releasable means is a pressure releasable means.
 46. In a dual-plug-cementing apparatus wherein two annular liner wiper means are disposed in a liner at the end of an operating string and wherein said two wipers are adapted to be successively engaged and released from the operating string by two plug means to form movable cement seals across the liner in front of and behind the cement, the improvement comprising: fluid seal means disposed between the operating string and the liner above the upper of said two wiper means to prevent upward flow past said fluid seal means between the operating string and the liner, and means to equalize the pressure within the annular volume defined by the operating string and the liner between the lower of said two wiper means and said fluid seal means with the pressure within the operating string to prevent pressure buildup within said annular volume from prematurely releasing said wiper means.
 47. The apparatus of claim 46 wherein said Means to equalize pressure comprises: holding means to prevent the wiper blades of said upper wiper means from forming a fluid seal with the liner; and port means to enable pressure equalization between said annular volume and the interior of said operating string.
 48. The apparatus of claim 47 wherein said holding means is a sleeve affixed to the operating string and is adapted to compress the said upper wiper to hold said wiper out of contact with the liner, and wherein said sleeve has pressure equalization ports to prevent pressure buildup between said upper wiper means and said sleeve. 